How do I calculate cable size for voltage drop?

Use the formula: CM = (K × I × L × multiplier) / VD. Where K is resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, L is one-way length in feet, multiplier is 2 for single-phase or 1.732 for three-phase, and VD is the maximum allowed voltage drop in volts.

What is the maximum allowed voltage drop?

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% for feeder circuits combined. Most applications use 3% as the default maximum voltage drop for branch circuits.

What size cable do I need for 100 feet?

For 20 amps at 240V single-phase with copper over 100 feet, you typically need 10 AWG copper (10,380 CM). The voltage drop would be approximately 5.6V (2.3%), which passes the 3% code requirement. Always use this calculator for your specific values.

What is the difference between AWG and mm² cable sizes?

AWG (American Wire Gauge) is the North American standard for wire sizes. mm² refers to the cross-sectional area in square millimeters, which is the metric standard. Larger AWG numbers mean smaller wire; larger mm² numbers mean larger wire. For example, 10 AWG ≈ 5.26mm².

Cable Voltage Drop Calculator

Determine the minimum cable size required to stay within voltage drop limits. Supports AWG and metric sizes, copper and aluminum, single and three phase.

Voltage (V)

Current / Load (amps)

One-Way Length (feet)

Cable Material

Phase

Max Voltage Drop (%)

10 AWG

5.26 mm²

✅ PASS — Within Code

Actual Voltage Drop

2.3%

Voltage Drop (Volts)

5.6 V

Max Allowed Drop

7.2 V (3.0%)

Cable Cross Section

10,380 CM

How to Calculate Cable Size

The formula and steps to find the right wire gauge for your voltage drop requirements.

The Voltage Drop Formula

VD = (K × I × L × M) / CM → CM = (K × I × L × M) / VD

Where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way length in feet, M is the phase multiplier (2 for single-phase, 1.732 for three-phase), and CM is the circular mil area of the conductor.

Step by Step

Calculate max allowed drop: Voltage × (Max Drop % / 100)
Find required circular mils: (K × I × L × M) / max drop in volts
Select standard cable: Choose the next larger standard AWG or mm² size
Verify: Calculate actual voltage drop with the selected cable

Voltage Drop Code Requirements

NEC recommendations for maximum voltage drop in electrical installations.

Branch Circuits

Maximum 3% voltage drop for branch circuits supplying lighting, outlets, and appliances. This is the most common standard.

Feeder Circuits

Maximum 2% voltage drop for feeder conductors. Combined feeder + branch drop should not exceed 5% total.

Critical Equipment

Sensitive electronic equipment may require tighter limits (1-2%). Motor circuits have special starting voltage drop considerations.

Cable Size Chart

Standard AWG and metric cable sizes with circular mil areas.

AWG	mm²	Circular Mils	Typical Use
14	2.08	4,110	Lighting, outlets (15A)
12	3.31	6,530	General purpose (20A)
10	5.26	10,380	Appliances, dryers (30A)
8	8.37	16,510	Ranges, HVAC (40-50A)
6	13.30	26,240	Sub-panels, large loads (55-65A)
4	21.15	41,740	Feeders, service entrances (70-85A)
2	33.62	66,360	Main feeders, large motors (95-115A)
1	42.41	83,690	Service conductors (130A)
1/0	53.49	105,600	Large service entrances (150A)
2/0	67.43	133,100	Heavy industrial (175A)

Frequently Asked Questions

Common questions about cable voltage drop calculations.

Use the formula: CM = (K × I × L × M) / VD. K is 12.9 for copper or 21.2 for aluminum. I is current in amps. L is one-way length in feet. M is 2 for single-phase or 1.732 for three-phase. VD is the maximum allowed voltage drop in volts.

The NEC recommends 3% maximum for branch circuits and 5% total for feeder + branch circuits combined. Some local codes may have stricter requirements. For sensitive equipment, 1-2% is often recommended.

Longer cable runs have higher resistance, which causes more voltage to be lost as heat. This is why longer runs require thicker cables — a larger cross-section reduces resistance and keeps voltage drop within acceptable limits.

AWG (American Wire Gauge) is the North American standard. Higher AWG numbers mean thinner wire. mm² is the metric cross-sectional area standard. For example, 10 AWG = 5.26 mm². This calculator shows both for convenience.

Copper has lower resistivity (12.9 vs 21.2 for aluminum), meaning less voltage drop for the same cable size. Copper is preferred for long runs where voltage drop is a concern. Aluminum is lighter and cheaper but requires one to two sizes larger to match copper's performance.